Hi Kevin,
I am not aware of a means to apply rich-text attributes to combobox entries. I would probably consider using a DISPLAY ARRAY comprised of a column of Labels (or TextEdits) with textFormat="html". So you'd populate the array similar to ...
LET arr[1].colour = '<div style="background-color:#FF0000">red</div>'
LET arr[2].colour = '<div style="background-color:#00FF00">green</div>'
LET arr[3].colour = '<div style="background-color:#0000FF">blue</div>'
... that gives you flexibility for background and foreground colours as well as 65536 colours to choose from.
Reuben
PS I'd also ask Pierre-Nicolas nicely if the colour picker dialog as used in Genero Studio Report Designer is something that could be called in a ui.Interface.frontCall() similar to the openFile,saveFile dialogs etc.